Introduction to Cast Iron: History, Types, Properties, and Uses

A versatile metal, cast iron has many unique applications in the commercial and industrial world

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The presence of iron in everyday life began in about 1200 BCE, encompassing a wide range of uses from farming
implements to weapons of war. Blacksmiths became a critical profession, working with iron to change its properties
and shape the material into tools. Every village and town would have a blacksmith’s shop, where sickles,
plowshares, nails, swords, candlestick holders, and more were produced.

The discovery of iron’s value led to what has become known as the Iron Age, due to the dominance of this material in
social and military applications. Another milestone for metals would follow—the Industrial Revolution changed the
way metals were produced and worked into products, including iron.

Types of iron

There are two major types of iron produced: wrought iron and cast iron.
Within those, cast iron includes its own family of metals.

Wrought iron

The first type of iron produced and worked by blacksmiths was wrought iron. It is virtually pure elemental
iron (Fe)
that is heated in a furnace before being wrought (worked) with hammers on an anvil. Hammering iron expels
most of
the slag from the material and welds the iron particles together.

During the industrial revolution and the associated acceleration of construction activities, a new use for
wrought
iron was discovered. Its high tensile strength (resistance to breaking when under tension) made it ideal to
use for
beams in large construction projects such as bridges and high-rise buildings. However, the use of wrought
iron for
this purpose was largely abandoned in the early 20th century when steel products were developed with
superior
performance to iron for construction applications.

Wrought iron has become famous for decorative pieces. Churches of the 15th and 16th century contain fine
wrought
iron pieces produced by skilled artisans. In the modern world, railings, doors, and benches are still made
from
wrought iron as custom pieces.

Cast iron

Cast iron is a family of metals produced by smelting metal, and then pouring it into a mold. The primary
difference
in production between wrought iron and cast iron is that cast iron is not worked with hammers and tools. There
are
also differences in composition—cast iron contains 2–4% carbon and other alloys, and 1–3% of silicon, which
improves the casting performance of the molten metal. Small amounts of manganese and some impurities like
sulfur
and phosphorous may also be present. Differences between wrought iron and cast iron can also be found in the
details of chemical structure and physical properties.

Due to the presence of carbon in cast iron, it may sometimes be confused
with steel. However, there are significant differences. Steel contains less than 2% carbon, which
enables
the final product to solidify in a single microcrystalline structure. The higher carbon content of cast iron
means
that it solidifies as a heterogeneous alloy, and therefore has more than one microcrystalline structure present
in
the material.

It is the combination of high carbon content, and the presence of silicon, that gives cast iron its excellent
castability.

Gray iron

Gray iron is characterized by the flake shape of the graphite molecules in the metal. When the metal is fractured,
the break occurs along the graphite flakes, which gives it the gray color on the fractured metal's surface. The
name gray iron comes from this characteristic.

It is possible to control the size and matrix structure of the graphite flakes during production by adjusting the
cooling rate and composition. Gray iron is not as ductile as other forms of cast iron and its tensile strength is
also lower. However, it is a better thermal conductor and has a higher level of vibration damping. It has a damping
capacity that is 20–25 times higher than steel and superior to all other cast irons. Gray iron is also easier to
machine than other cast irons, and its wear resistance properties make it one of the highest volume cast iron
products.

White iron

With the right carbon content and a high cooling rate, carbon atoms combine with iron to form iron carbide. This
means that there are little to no free graphite molecules in the solidified material. When white iron is
sheared,
the fractured face appears white due to the absence of graphite. The cementite microcrystalline structure is
hard
and brittle with a high compressive strength and good wear resistance. In certain specialized applications, it
is
desirable to have white iron on the surface of the product. This can be achieved by using a good conductor of
heat
to make part of the mold. This will draw heat out of the molten metal quickly from that specific area, while
the
rest of the casting cools at a slower rate.

One of the most popular grades of white iron is Ni-Hard Iron. The addition of chromium and nickel alloys gives
this
product excellent properties for low impact, sliding abrasion applications.

Malleable iron

White iron can be further processed into malleable iron through a process of heat treatment. An extended program of
heating and cooling, results in the breakdown of the iron carbide molecules, releasing free graphite molecules into
the iron. Different cooling rates, and the addition of alloys, produces a malleable iron with a microcrystalline
structure.

Ductile iron (Nodular iron)

Ductile iron, or nodular iron, obtains its special properties through the
addition of magnesium into the alloy. The presence of magnesium causes the graphite to form in a spheroid shape as
opposed to the flakes of gray iron. Composition control is very important in the manufacturing process. Small
amounts of impurities such as sulfur and oxygen react with the magnesium, affecting the shape of the graphite
molecules. Different grades of ductile iron are formed by manipulating the microcrystalline structure around the
graphite spheroid. This is achieved through the casting process, or through heat treatment, as a downstream
processing step.

Compacted graphite iron

Compacted graphite iron has a graphite structure and associated properties that are a blend of gray and white iron.
The microcrystalline structure is formed around blunt flakes of graphite which are interconnected. An alloy, such
as titanium, is used to suppress the formation of spheroidal graphite. Compacted graphite iron has a higher tensile
strength and improved ductility compared to gray iron. The microcrystalline structure and properties can be
adjusted through heat treatment or the addition of other alloys.

Summary of cast iron compositions

A table developed by the Engineer’s
Handbook shows the different composition ranges for the various types of cast iron:

Mechanical properties of cast iron

The mechanical properties of a material indicate how it responds under specific stresses, which helps to determine
its suitability for different applications. Specifications are set by organizations such as the American Society for Testing and Materials (ASTM) so that users can purchase materials with
confidence that they meet the requirements for their application. The most commonly used cast gray iron
specification is ASTM A48.

In order to qualify cast products according to their specifications, a standard practice is to cast a test bar along
with the engineered castings. The ASTM tests are then applied to this test bar and the results are used to qualify
the entire batch of castings.

Specifications are also important when welding cast iron parts together.
The weld must meet or exceed the mechanical properties of the material being welded together—otherwise, fractures
and failures can occur.

A few common mechanical properties for cast iron include:

Hardness – material's resistance to abrasion and indentation

Toughness – material’s ability to absorb energy

Ductility – material's ability to deform without fracture

Elasticity – material's ability to return to its original dimensions after it has been deformed

Malleability – material's ability to deform under compression without rupturing

Fatigue strength – the highest stress that a material can withstand for a given number of cycles without
breaking

This table summarizes some of the key mechanical properties for various grades of cast iron. For more information,
see “Iron Alloys”, a great reference document
from the American Foundry Society.

Brinell Hardness

Tensile Strength

Modulus of Elasticity

% Elongation (in 50 mm)

Gray iron class 25

187

29.9 ksi

16.6 Msi

-

Gray iron class 40

235

41.9 ksi

18.2 Msi

-

Ductile iron grade 60-40-18

130-170

60 ksi

24.5 Msi

-

Ductile iron grade 120-90-02

240-300

120 ksi

25.5 Msi

-

CGI grade 250

179 max

36.2 ksi min

3

CGI grade 450

207-269

65.2 ksi min

1

Common applications of cast iron

The various properties of different types of cast iron results in each type being suited for specific applications.

Gray iron applications

One of the key characteristics of gray iron is its ability to resist wear even when lubrication supply is limited
(e.g. the upper cylinder walls in engine blocks). Gray iron is used to make engine blocks and cylinder heads,
manifolds, gas burners, gear blanks, enclosures, and housings.

White iron applications

The chilling process used to make white iron results in a brittle material that is very resistant to wear and
abrasions. For this reason, it is used to make mill linings, shot-blasting nozzles, railroad brake shoes,
slurry
pump housings, rolling mill rolls, and crushers.

Ductile iron applications

Ductile iron itself can be broken down into different grades, each with their own property specifications and most
suitable applications. It is easy to machine, has good fatigue and yield strength, while being wear resistant. Its
most well-known feature, however, is ductility. Ductile iron can be used to make steering knuckles, plow shares,
crankshafts, heavy duty gears, automotive and truck suspension components, hydraulic components, and automobile
door hinges.

Malleable iron applications

Different grades of malleable iron correspond to different microcrystalline structures. Specific attributes that
make malleable iron attractive are its ability to retain and store lubricants, the non-abrasive wear particles, and
the porous surface which traps other abrasive debris. Malleable iron is used for heavy duty bearing surfaces,
chains, sprockets, connecting rods, drive train and axle components, railroad rolling stock, and farm and
construction machinery.

Compacted graphite iron applications

Compacted graphite iron is beginning to make its presence known in commercial applications. The combination of the
properties of gray iron and white iron create a high strength and high thermal conductivity product—suitable for
diesel engine blocks and frames, cylinder liners, break discs for trains, exhaust manifolds, and gear plates in
high pressure pumps.

Machining and finishing

The hardness properties of cast iron demand careful selection of machine tool materials. Coated carbides are
effective in production machining environments, but newer materials are being developed continuously as technology
improves.

Surface finishing of cast iron products varies greatly according to the use. A few common applications:

Electroplating

Hot-dipping

Thermal spraying

Diffusion coating

Conversion coating

Porcelain enameling

Liquid organic coating

Dry powder organic coating

Cast iron and the future

From its early use over 3,000 years ago, iron has remained an integral part of human society. Iron production has
come a long way since the centuries of working iron by blacksmiths to the invention of cast iron in the industrial
age.

Since then, wrought iron has become largely obsolete except for decorative uses. Contrastingly, cast iron is still
progressing in terms of composition, microstructure, and mechanical properties—continuing to make its mark in the
modern world.